The present invention relates to rotary plug valves and, in particular, to plug valves in which a housing is molded in place about a preformed rotary plug.
Rotary plug valves are conventional in which a housing forms a through-passage and rotatably mounts an inner plug or gate, e.g., a ball, situated in the through-passage. The ball has a bore therethrough which, when aligned with the through-passage, admits a flow of fluid and, when misaligned prevents such flow. It has heretofore been proposed to form such a valve by preforming the ball member, as from a thermoplastic material for example, placing that ball in a mold and then molding a thermoplastic housing material in-place around the ball (see for example commonly assigned U.S. Pat. No. 4,257,575, and U.S. Pat. Nos. 3,961,770; 3,271,845; and 3,807,692 for disclosures of valves formed in this manner). A pair of seating rings may or may not be placed in the mold around the housing through-passage prior to injection of the housing material.
A disadvantage which arises during such a fabrication technique results from the relatively high temperature of the molten housing material as it is flowed into the cavity surrounding the preformed ball. Unless the ball is formed of a material sufficiently resistant to that heat level, the ball may become welded to the housing, especially in the region where the molten housing is introduced since temperatures are highest there. In many instances, this problems dictates that the ball be formed of a different material than the housing (i.e., of a more heat resistant material). However, if the ball and housing comprises different materials, it may be difficult to provide a valve whose components exhibit common characteristics, such as the same chemical resistance for example. For this reason it may be difficult, with such a technique to provide a valve which is capable of being employed in certain chemically corrosive environments.
It should also be mentioned that even in cases where the ball is formed of a material having a higher temperature resistance than the housing material in order to avoid the above-described welding problem, the high temperatures in the region where molten housing material is introduced may still cause damage to the ball.
It is, therefore, an object of the present invention to minimize or obviate problems of the type discussed above.
Another object of the present invention is to provide novel methods and apparatus for molding a valve housing around a preformed plug while minimizing the risk of heat-induced damage to the plug when introducing molten housing material therearound.
An additional object of the invention is to provide such methods and apparatus while assuring that no leakage occurs around the plug during usage of the valve.
An additional object of the present invention is to enable the plug to be formed of a material having a heat-resistance which is equal to, greater than, or less than that of the housing material.